An image processing by so-called stereo method is generally known as a 3-dimensional measurement technology using imaging. This method employs 2 cameras (stereo cameras) to take a pair of images of an object from different positions, correlate the pair of images to thereby obtain information on a distance to the object with the triangulation principle using camera parameters such as stereo camera positions, a focal length, and the like from a parallax for the same object.
In image processing employing the above stereo method, a disparity between the 2 images is detected by determining a position where 2 sets of image signals from the stereo cameras match while sequentially shifting the 2 sets of image signals to thereby calculate a distance to the object. In general, the above position where 2 sets of image signals match is obtained by comparing brightness of the image signals from the 2 cameras.
This image comparison is performed for each small area with a predetermined pixel count due to a computation speed and the like. For a base image and a reference image, for example, a city block distance is calculated to correlate each other for each small area (also called a “matching block”) of each image to thereby perform stereo matching processing for identifying corresponding areas to obtain 3-dimensional image information (a distance image), which is a numeric expression of information on a distance to the object obtained from a pixel disparity (=parallax) according to a distance to the object.
In the stereo matching processing for the 2 images, however, a mismatch problem may occur where the correspondence is incorrect.
To address this mismatch problem, JP-A-2001-160144 discloses an apparatus comprising a mechanism for determining the existence of the mismatch from a result obtained in the image matching processing.
Also JP-A-H11-248446 discloses a technique for voting on the parallax for the matching block in question and its surrounding 8 blocks to reduce the mismatch.
Further, JP-A-H8-279045 discloses a technique for specifying a priority for a tentative corresponding point candidate against other corresponding points, and obtaining the parallax consistent with the correspondence.
However, the above JP-A-2001-160144 technique simply determines the presence of the mismatch and a mechanism for reducing a mismatch influence to improve the stereo matching accuracy is not disclosed.
Moreover, the proposal in the JP-A-H11-248446 does not offer sufficient effect when actual space has 3-dimensional depth equivalent to a resolution of the matching block, or for areas such as a wall surface without sufficient image features for the matching processing.
Likewise, the proposal in the JP-A-H8-279045 does not provide enough correction effect for areas such as a wall surface.